Italy itinerary - arriving June 17th

I'm going to suggest, based on my little knowledge of that region, that most of what you have listed isn't really considered northern Italy but Tuscany.

As for the area around Milan, obviously the lake district, perhaps Verona, Vicenza, even Venice? And then on the way to Florence, perhaps Bologna?

You have 7 locations already planned for a 17 night trip
Honestly that is enough
3 nights Varenna
3 Dolomites ( this is an outlier)
3 CT
Maybe 2 nights in LUCCA (or could be day trip from Florence)

End with remaining nights in Florence (since that is your departure airport)with day trips to Siena, SG (and possibly Lucca)

What do you like to do? Or, put another way, why did you pick these locations and what do you plan to do in each? We want to help you plan but how much time in each location depends on what you will do when there. Museums? Hiking? Wine Tours?

A few thoughts pending answers to the above:

• You cannot get to San Gimignano by train. Car OR train plus bus.
• You can visit Siena and Lucca by train as day trips while staying in Florence, depending on what you want to do i Siena and Lucca.
• The Dolomites encompasses a huge area, the opposite direction of Tuscany and the Cinque Terre from your starting point in Varenna. Any idea exactly where in the mountains you’d like to go?

Thanks for your replies. We like to explore, hike, eat, experience the culture, meet locals, and see the sights. We will go to a few museums, but not too many. I know we don't have time to see all of the Dolomites, but I would like to stay in Ortisei and explore from there and maybe one other area. I still need to do more research.
Do you suggest doing day trips from Florence or is there a smaller town worth spending 2 nights in? I do like the suggestion of Lucca.
We have already been to Venice, but Bologna might worth stopping to eat! :)

Here’s what I suggest:
Varenna – three nights. Bellagio is a good day trip from Varenna.
Cinque Terre – sleep in Vernazza. If you’re not hiking you only need two nights. If you are hiking add a night per hike.
Florence – three nights. Buy tickets for the Uffizi Gallery two months out to guarantee a reservation. Afterwards go see Michelangelo’s David at Accademia. A good day trip from Florence is Lucca (2h by direct train).
Siena – take a direct bus from Florence to Siena that will drop you off in the city center, the train will drop you off elsewhere; sleep in Siena for two nights. The next day take a direct bus from Siena to San Gimignano (1h 15m) for the day.
If you’re up to it rent a car in Siena and drive to the Dolomites, otherwise it'll take all day to get there by train. If you do rent a car, make sure you read about the ZTL zone laws.

I like MaryPat's basic itinerary for you but would order it this way since you fly out of Florence:

Varenna - 3 nights

Cinque Terre - 3 nights (NOTE: This is a 5+hour train trip so staying for 3 nights makes a lot of sense)

Ortisei/Val Gardena - at least 4 nights (Rent a car in La Spezia. Expect a 5 hour drive to stay at least 4 nights. You will not regret it.)

Siena - 2 or 3 nights with a day trip to San Gimignano (Another 5-ish hour drive; if you can find parking with your lodging, keep the car until you have been to San Gimignano to make that a easy day trip)

Florence - 3 or 4 nights with a day trip by train to Lucca

More time in the mountains makes sense for multiple reasons: Fickle weather, the investment in time to get there, and because no one I have ever spoken with wished they spent less time in the Val Gardena.